Some requirements for modern batteries include high energy density (product of capacity and voltage) to reduce weight and/or space, the capacity for numerous charge/discharge cycles to enable longer life, and little or no memory effects so that later cycles provide similar electrical potential as earlier cycles when the battery is new.
Lithium-sulfur batteries have one electrode made of lithium and another made of sulfur. But sulfur is an insulating material (sulfur alone being at 5*10−30 S cm−1 at 25° C.), so to improve conductivity in some battery implementations the sulfur electrode is compounded with carbon to act as the battery cathode. As with the conventional lithium ion batteries, charging and discharging the battery involves the movement in an electrolyte of lithium ions between the two electrodes.
The theoretical capacity of lithium-sulfur batteries is much higher than that of lithium-ion batteries because of the way the ions are assimilated at the electrodes. For example, at the sulfur electrode, each sulfur atom can host two lithium ions, while in lithium-ion batteries every host atom can accommodate only 0.5 to 0.7 lithium ions.
Making materials that take advantage of the higher theoretical capacity of lithium-sulfur (Li—S) battery has been a challenge. The fact that sulfur is an insulating material makes it difficult for electrons and ions to move in and out of capture at the sulfur electrode. So while each sulfur atom may in theory be able to host two lithium ions, in fact often only those atoms of sulfur near the surface of the material accept lithium ions. Another problem is that as the sulfur binds to lithium ions and eventually forms dilithium sulfide, it also forms a number of intermediate products called polysulfides. These dissolve in the battery's liquid electrolyte and eventually can settle in other areas of the battery, where they can block charging and discharging. Because of this, prior art lithium sulfur batteries can stop working altogether after only a few dozen cycles.
In traditional lithium ion batteries, the charge storage capability is inherently limited to about 300 mA−h/g, and to the inventor's knowledge the maximum capacities observed are about 180 mA−h/g with high power characteristics. A lithium sulfur battery cell operates quite differently from a lithium ion battery cell. Specifically, the redox couple, which in general is the tendency of a chemical species to reduce by acquiring electrons and which specifically for lithium sulfur batteries is described by the discharge reaction S8→Li2S8→Li2S6→Li2S4→Li2S3→Li2S2→Li2S. Polysulfides are reduced on the anode surface in sequence while the cell is discharging:S8→Li2S8→Li2S6→Li2S4→Li2S3 Across a porous diffusion separator, the polymers of sulfur are formed at the nominal cathode as the cell charges:Li2S→Li2S2→Li2S3→Li2S4→Li2S6→Li2S8→S8 This redox couple lies near 2.2 V with respect to Li+/Li, a potential which is only about ⅔ of that exhibited by conventional positive electrodes. However, this is offset by the very high theoretical capacity afforded by the non-topotactic ‘assimilation’ process of 1675 mAh/g.
Specifically, the chemical processes in the Li—S battery cell include lithium dissolution from the anode surface (and incorporation into polysulfides) during discharge, and lithium plating back on to the nominal anode while charging. This contrasts with conventional lithium-ion cells in which the lithium ions are intercalated in the anode and cathodes, and this distinction allows the Li—S arrangement to exhibit, in theory, a much higher lithium storage density. Compared with intercalation batteries such as lithium-ion types, Li—S cells have the opportunity to provide a significantly higher energy density. Values can approach 2,500 Wh/kg or 2,800 Wh/l on a weight or volume basis respectively, assuming complete reaction to Li2S.
In practice, the various problems noted above hinders the complete reaction to form Li2S. However, the performance of current commercial Li—S batteries is still higher than conventional lithium ion batteries as shown in FIG. 1, at least in the mass energy density category which is plotted along the vertical axis. Improvements to Li—S technology is expected to follow the dashed line in which Li—S would also outperform lithium ion batteries also in the volume energy density category shown along the horizontal axis of FIG. 1.
To date, various carbon-sulfur composites have been used to improve the Li—S battery performance, but they have limitations owing to the scale of the contact area. Typical reported capacities are between 300 and 550 mA−h/g at moderate rates, such as are described at P. T. Cunningham, S. A. Johnson, and E. J. Cairns, J. ELECTROCHEM. SOC., 119 (1972) 1448. In response to many considerable challenges, novel advances in materials design such as new electrolytes [see for example J. H. Shin and E. J. Cairns, J. ELECTROCHEM. SOC., 155 (2008) A368] and protective films for the lithium anode [see for example K. I. Chung, W. S. Kim, and Y. K. Choi, J. ELECTROANAL. CHEM., 566 (2004) 263] have been developed. Combinations of electrolyte modification, additives and anode protection have resulted in some promising results according to J. R. Akridge, Y. V. Mikhaylik, and N. White, SOLID STATE ION, 175 (2004) 243. Much of the difficulty still remains at the cathode, where the lack of breakthroughs has led to some cell configurations in which all of the sulphides are solubilised. More recently, it has been demonstrated that cathodes based on sulfur/mesoporous carbon materials can overcome these challenges to a large degree, and exhibit stable, high, reversible capacities (up to 1,320 mAh/g) with good rate properties and cycling efficiency [see for example X. Ji, K. T. Lee, and L. F. Nazar, NATURE MATERIALS, 8 (2009) 500].
Lithium-sulfur batteries, which can potentially store several times more energy than lithium ion batteries, have historically been too costly, unsafe and unreliable to make commercially. As will be shown in the exemplary embodiments below, improvements to the design of these batteries by nanotechnology can overcome such problems and bring the much more enhanced energy density Li—S batteries to the portable electronics as well as high-energy requested applications such as electric vehicles.